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Regarding the use of a non Shimano crank with Di2.I just came back from InterBike where I asked the same question to several people.At Shimano they said: You must use a 7900 Crank with Di2, anything else will be almost unusable. The reason being is that the chain-rings, according to this Shimano guy, are about 2mm wider than all other crank sets. For example, if a standard crank has it's chainrings 4mm apart (just a guess) than the 7900 chainrings are somewhere between 5.5-and 6mm apart.He said what will happen if you use a non 7900 Crank is that when the front derailleur "auto-trims" it will kick the chain off the big chain ring, back to the small chain ring.This bummed me out as I had my heart set on a light weight carbon crank set and wanted to use it with Di2.

There were several "Show" bikes with Di2 and cool non-Shimano cranks installed, but those bikes were all new with no miles on them.No one else could confirm or deny this information I got from ShimanoUntil . . .I went to the FairWheel bike booth. Super cool guys there with some of the best stuff in the show.They have been using Di2 with different cranks with no problems. They did have a word or two of caution.They said that the Di2 FD is so strong that it can easily break or bend your big chain-ring if you aren't careful. This can happen when shifting to the big Chain-ring while peddling very slow as the FD will basically force the chain up the the big chain-ring. The 7900 crank has super robust chain-rings so there is no problem, but in the after market arena you must use very strong chain-rings, at least for the big chain-ring.But it was encouraging to hear that they have had success with other cranks and using Di2.That supports some of the information in this thread but I wanted to share what I learned while at InterBike.

The bit about auto trim pulling the chain off a chainring is absurd. The chainrings are not materially different in dimension than non DA rings. Further, the auto trim moves the FD no more than we moved the FD in pre indexed days.

The real problem, what I heard from reliable sources, is that the FD servo-motor is so strong than it could slightly bend the big ring upon every shift. If the big chainring isn't stiff/strong enough, this could potentially lead to chainring failure. With the use of a Dura-Ace 7800 or 7900 crankset, this potential hazard is virtually eliminated.

“I always find it amazing that a material can actually sell a product when it’s really the engineering that creates and dictates how well that material will behave or perform.” — Chuck Teixeira

This makes no sense. Sure the motor is strong, but I can't see how it puts any more stress on the chainrings than a manual derailleur - indexed or not. Think how easy the shifting action is with a non-indexed derailleur - how is the chainring stressed any differently with a motor pushing the FD?

how is the chainring stressed any differently with a motor pushing the FD?

I suppose it could push hard on the big ring IF it is improperly adjusted and is too low and actually contacts the big ring. If the FD is simply moving the chain, I agree that it can't create any more stress on the big ring than any other manual system.

I'm using a FSA K light compact with my Di2 with no issues.The mech is simply amazing, don't worry yourself about if it might bend the ring. Under normal use i can't see this happening.By the way, I have both DA chainset and FSA, and i think the change is much cleaner with the FSA using a KMC sl.

marchitect wrote:The reason being is that the chain-rings, according to this Shimano guy, are about 2mm wider than all other crank sets. For example, if a standard crank has it's chainrings 4mm apart (just a guess) than the 7900 chainrings are somewhere between 5.5-and 6mm apart.

Interesting. A couple months ago Shimano customer service told me the 7900 chain ring spacing hasn't changed from 7800. FSA also told me the same thing (i.e. their chain ring spacing is the same as 7900).